Cycles Render Equivalent of "X-Ray" Function
You can move liguid to second layer and create second render layer. After merge them in compositor.



In this case you can't use Material ID or Object ID, because liquid is in other object, or suzanne in cube in my case. But is good to know there is this possibility. 
Workflow: use "Pass Index" (material or object) for create mask and after use this mask to separate object in compositor.